To select a replacement for a broken or worn out
hydraulic pump or motor which has no nameplate or has no rating marked
on its case, use the formulas below after making internal physical measurements.

When replacing a pump, catalog ratings will usually
be shown in GPM at a specified shaft speed. On a motor, catalog ratings
will usually be in C.I.R. (cubic inches per revolution). Formulas are given
for calculating either GPM at 1800 RPM or calculating C.I.R. Use the formula
which is appropriate. Make all measurements in inches, as accurately as possible.
Convert fractional dimensions into decimal equivalents for use in the formulas.

Make sure the catalog pressure rating is adequate for your
application, and in the case of a pump, be sure direction of shaft rotation is correct.

A speed of 1800 RPM is used in the formula.
At other speeds, GPM is proportional to RPM.

A speed of 1800 RPM is used in the formula.
At other speeds, GPM is proportional to RPM.

A speed of 1800 RPM is used in the formula.
At other speeds, GPM is proportional to RPM.

C.I.R. Displacement =

C.I.R. Displacement =

C.I.R. Displacement =

6 x W x (2D - L) x

(L - D)

2

A x L x N

12 x W x

(L + D)

x

(L - D)

4

2

If a pump of higher GPM has to be used, it will
require more HP at the same pressure and cylinders in the system will
move faster. If one with lower GPM is used, the system will have plenty
of power but cylinders will move more slowly than originally.

If a motor with greater displacement is used,
it will deliver more torque at a reduced RPM, but will require no more
fluid HP from the pump. If it has less displacement it will rotate faster with less torque.